The present invention relates generally to particle classification. More particularly, the present invention relates to the classification of particles from a fluid stream via centrifugal separation imparted by boundary layers developed between rotating parallel disks, whereby the primary airflow runs counter to the ejected trajectory of the particles larger than the cut-size of the device. Fine particles smaller in diameter than the cut-size of the classifier follow the primary airflow streamlines to a collection vessel.
Air classifiers can be used to classify, or selectively filter, airborne particles according to their size, density, or shape based on a balance between inertial and fluid forces acting on the particles in a flow field. Air classifiers are used primarily to produce particles in a limited size range. Production of powders in a limited size range is important in many different types of industries (e.g., electronics, food, chemical, petroleum, and pharmaceuticals), where product quality depends on the size distribution of the powder used to make the product. Size distribution affects powder properties such as homogeneity, flowability, taste, texture, and surface area. A narrow size distribution can give the final product more uniformity to improve product quality. Industries commonly use air classifiers to classify dry powders because they can handle higher mass throughputs than other methods such as screening. In the production of fine powders, air classification often follows a comminution, or grinding, process. Powder generated from the grinding process is sent to an air classifier to remove the coarser particles.
Most air classifiers separate the feed material into two size groups: a coarse fraction and a fine fraction. Separation is typically measured by efficiency, which can be generally defined as the fraction of particles of a given size (d) in the feed reporting to the coarse fraction,                               Efficiency          ⁢                      xe2x80x83                    ⁢                      (            d            )                          =                              f            c                    ⁢                      xe2x80x83                    ⁢                                                    q                c                            ⁢                              xe2x80x83                            ⁢                              (                d                )                                                                    q                F                            ⁢                              xe2x80x83                            ⁢                              (                d                )                                                                        (1-1)            
where fc is the overall efficiency, or the overall mass fraction of particles in the feed reporting to the coarse fraction, and qC(d) and qF(d) are the differential size distributions of the coarse fraction and feed, respectively. Cut size is most often defined as the particle size with 50% efficiency (d50), efficiency (d =d50)=0.50.
The ideal efficiency curve is a unit step function at the cut size. That is, all particles below the cut size leave with the air in the fine fraction, while all particles above the cut size are collected from the air as the coarse fraction. Theoretically, particles at the cut size should be suspended in the classification zone since the forces acting on the particles are balanced. In reality, 50% of particles at the cut size leave with the coarse fraction, and 50% leave with the fine fraction. Air classifiers do not give an ideal separation of particles at the cut size because of random fluctuations in the flow field, particle-particle interactions, and variations in the forces acting on the particles in relation to the position of the particles in the classifier. The quality of the separation, or cut sharpness, is commonly represented by the values K25/75 and K10/90, which are respectively defined as:
K25/75=d25/d75xe2x80x83xe2x80x83(1-2)
K10/90=d10/d90xe2x80x83xe2x80x83(1-3)
where d10, d25, and d90 are the particle diameters with 10, 25, 75 and 90% efficiency, respectively. Under normal circumstances, K25/75 and K10/90 can have values between 0 and 1, where 1 represents an ideal separation.
The extent to which particles are dispersed in air contributes to cut sharpness. To accurately classify a powder, each particle must travel through the flow field independent of other particles. Therefore, to minimize the amount of particle agglomerates entering the classifier, the feed should be dispersed before classification. The energy necessary to disperse a powder can be supplied mechanically (e.g., a mixing blade) or by shear force (e.g., feeding the particles into a high-velocity air stream). Since agglomeration is often caused by moisture in the air, the relative humidity of the air should be sufficiently low. However, if humidity is too low, the particles can become electrostatically charged, which also hinders dispersion. The optimum relative humidity for classification is approximately 50 to 70%. Particle size also affects dispersion, since attractive forces between particles increase as particle size decreases. Because of the difficulty in dispersing very fine particles, especially below 1 xcexcm, these particles are commonly collected with the coarse fraction as agglomerates. As solids loading, or particle concentration, increases, adequate dispersion becomes more difficult. Consequently, cut sharpness must be compromised for a higher particle loading. This is a dilemma for industries, which need to classify particles at high throughputs to minimize energy consumption.
The lower limit on cut size for air classifiers is typically 1 or 2 xcexcm. For particles less than 1 xcexcm, extremely high tangential velocities are required for the inertial force to overcome the fluid forces on the particles. Random particle motion caused by diffusion and Brownian motion also becomes significant below 1 xcexcm. See Leschonski, K., xe2x80x9cClassification of particles in the submicron range in an impeller wheel air classifier,xe2x80x9d KONA, No. 14, pp. 52-60 (1996). This is more of a problem if the flow is turbulent. Submicron cut sizes have been achieved by air classification, but only for limited throughputs. In addition, achieving sharp cuts at these small cut sizes is a challenge because of the difficulty in dispersing submicron particles.
Separation of particles from the air stream can be either counterflow or crossflow. In counterflow classifiers, particles are removed from the fluid in the direction opposite to the main flow. In crossflow classifiers, particles are removed perpendicular to the main flow. Particle removal is usually accomplished by gravity or inertial forces, such as centrifugal force, which isdue to the angular momentum of the flow. Because centripetal acceleration can be much stronger than gravitational acceleration at high tangential velocities, centrifugal classifiers are able to remove smaller particles from the flow than gravity-based classifiers. In general, counterflow classifiers can provide smaller cut sizes than crossflow classifiers, with the counterflow centrifugal classifier providing the smallest cut sizes. The approximate cut size range for counterflow centrifugal air classifiers is 1 to 100 xcexcm for mineral particle densities (xcx9c2000 to 3000 kg/m3). The only other type of classifier that has achieved cut sizes as small as the counterflow centrifugal classifier is the crossflow elbow classifier. See Maly, K., xe2x80x9cUntersuchung der partikel-strxc3x6mungsmittel-wechselwirkung im strahlumlenk-windsichter,xe2x80x9d Dissertation, Techn. Hochschule Karlsruhe, Germany (1979); Rumpf, H. and Leschonski, K., xe2x80x9cPrinzipien und neuere verfahren der windsichtung, Chemie-Ing.-Technik,xe2x80x9d Vol. 39, pp. 1231-1241 (1967). However, the crossflow elbow classifier cannot handle as high throughputs.
The basic principle of centrifugal counterflow classification can be understood from a force balance on the particle in the radial direction. For a particle in a rotating flow field, neglecting gravity and assuming particle density (xcfx81p) is much greater than fluid density (xcfx81), the equation of motion in the r-direction is given by:                                                         m              p                        ⁢                          xe2x80x83                        ⁢                                          ⅆ                                  v                                      r                    ,                    p                                                                              ⅆ                t                                              =                                                    F                D                            +                                                m                  p                                ⁢                                  xe2x80x83                                ⁢                                                      v                                          θ                      ,                      p                                        2                                    r                                                      =                                          F                D                            +                              F                C                                                    ;                              F            C                    =                                    m              p                        ⁢                          xe2x80x83                        ⁢                                          v                                  θ                  ,                  p                                2                            r                                                          (1-4)            
where mp is the particle mass, vr,p and vxcex8,p are the particle radial and tangential velocities, FD is the drag force, and FC is the centifugal force. For a spherical particle,
mp=xcfx80/6xcfx81pDp3xe2x80x83xe2x80x83(1-5)
where Dp is the particle diameter. Considering only particles in the Stokes, or xe2x80x9ccreeping,xe2x80x9d flow regime, FD is given by Stokes law:
FD=3xcfx80xcexcDp(vrxe2x88x92vr,p)xe2x80x83xe2x80x83(1-6)
where xcexc and vr are the fluid dynamic viscosity and radial velocity. Equation 1-6 generally holds for Rep less than 2 and Dp greater than 1 xcexcm. The relative particle Reynolds number (Rep) is given by:                               Re          p                =                              ρ            ⁢                          xe2x80x83                        ⁢                          D              p                        ⁢                          xe2x80x83                        ⁢                          "LeftBracketingBar"                                                v                  r                                -                                  v                                      r                    ,                    p                                                              "RightBracketingBar"                                μ                                    (1-7)            
Substituting Equations 1-5 and 1-6 into Equation 1-4 gives:                                           m            p                    ⁢                      xe2x80x83                    ⁢                                    ⅆ                              v                                  r                  ,                  p                                                                    ⅆ              t                                      =                              3            ⁢                          xe2x80x83                        ⁢            π            ⁢                          xe2x80x83                        ⁢            μ            ⁢                          xe2x80x83                        ⁢                          D              p                        ⁢                          xe2x80x83                        ⁢                          (                                                v                  r                                -                                  v                                      r                    ,                    p                                                              )                                +                                    π              6                        ⁢                          xe2x80x83                        ⁢                          ρ              p                        ⁢                          xe2x80x83                        ⁢                          D              p              3                        ⁢                          xe2x80x83                        ⁢                                          v                                  θ                  ,                  p                                2                            r                                                          (1-8)            
For counterflow centrifugal classification, FC must act opposite to FD. Since FC acts radially outward in the positive r-direction, the main fluid flow is radially inward, in the negative redirection, so that vr less than 0. Since FC is proportional to Dp3, while FD is only proportional to Dp, FC becomes the dominant force as Dp increases, making it more likely that the particle will be removed from the air. The theoretical cut size (Dpc) is determined by setting vr,xcfx81and the particle acceleration   (            ⅆ              v                  r          ,          p                            ⅆ      t        )
to 0 in Equation 1-8:                               D          pc                =                                            18              ⁢                              xe2x80x83                            ⁢              μ              ⁢                              xe2x80x83                            ⁢                              v                r                            ⁢                              xe2x80x83                            ⁢              r                                                      ρ                p                            ⁢                              xe2x80x83                            ⁢                              v                                  θ                  ,                  p                                2                                                                        (1-9)            
A particle of diameter Dpc will revolve around the axis of rotation at a constant radius.
In a centrifugal classifier, the flow can be either free or forced vortex. The difference between a free and forced vortex is in the dependence of the fluid tangential velocity (vxcex8) on radius (r). In a free vortex,                                           v            θ                    ⁢                      xe2x80x83                    ⁢                      (            r            )                          ∝                  1                      r            m                                              (1-10)            
where m=1 in an ideal free vortex. To account for friction losses, m is usually set a value between 0.5 and 0.9. In a forced vortex, m=xe2x88x921. For a non-accelerating particle of diameter Dpc, from the particle equation of motion in the tangential direction,                                           m            p                    ⁢                      xe2x80x83                    ⁢                      (                                          2                ⁢                                  v                                      r                    ,                    p                                                  ⁢                                  xe2x80x83                                ⁢                                                      v                                          θ                      ,                      p                                                        r                                            +                              r                ⁢                                  xe2x80x83                                ⁢                                                      ⅆ                                          xe2x80x83                                                                            ⅆ                    t                                                  ⁢                                  xe2x80x83                                ⁢                                  (                                                            v                                              θ                        ,                        p                                                              r                                    )                                                      )                          =                  3          ⁢                      xe2x80x83                    ⁢          π          ⁢                      xe2x80x83                    ⁢          μ          ⁢                      xe2x80x83                    ⁢                      D            p                    ⁢                      xe2x80x83                    ⁢                      (                                          v                θ                            -                              v                                  θ                  ,                  p                                                      )                                              (1-11)            
the particle tangential velocity is equal to the fluid tangential velocity,
vxcex8,xcfx81=vxcex8xe2x80x83xe2x80x83(1-12)
From Equations 1-10 and 1-12, in a free vortex, FC (Equation 1-4) is inversely proportional to r(1+2m), while in a forced vortex, FC is directly proportional to r. Therefore, in a free vortex, FC increases as r decreases; but in a forced vortex, FC decreases as r decreases.
In a counterflow centrifugal classifier, vr is generally given by the fluid volumetric flow rate (Q) divided by the cross-sectional area for flow, assuming vr is uniform in the axial and tangential directions,                                           v            r                    ⁢                      xe2x80x83                    ⁢                      (            r            )                          =                  Q                      2            ⁢                          xe2x80x83                        ⁢            π            ⁢                          xe2x80x83                        ⁢            τ            ⁢                          xe2x80x83                        ⁢            L                                              (1-13)            
where L is the height in the axial direction of the flow channel. If Equation 1-13 holds, FD (Equation 1-6) is inversely proportional to r. Substituting Equation 1-12 into Equation 1-9 and using Equations 1-10 and 1-13, one finds that in a free vortex,
Dpcxe2x88x9drm,0.5 greater than m greater than 1xe2x80x83xe2x80x83(1-14)
but in a forced vortex,
Dpcxe2x88x9d1/r
It has been shown that to obtain a sharp cut of particles in a counterflow centrifugal classifier, Dpc in the direction of flow (radially inward) must increase. See Rumpf, H. and Leschonski, K., xe2x80x9cPrinzipien und neuere verfahren der windsichtung,xe2x80x9d Chemie-Ing.-Technik, Vol. 39, pp. 1231-1241 (1967). Therefore, the flow must be a forced vortex.
To understand the advantages of having a forced vortex over a free vortex, consider a particle of diameter Dpc, which has an equilibrium radius (req) where |FC|=|FD| and vr,xcfx81=0. At req, the particle follows a trajectory of constant radius. If the particle encounters a small perturbation from req in the r-direction, the forces on the particle are no longer equal. In a free vortex, if the particle moves to a smaller r, Fc increases faster than FD returning the particle to req. Thus, in a free vortex, stable trajectories are formed, suspending particles in the flow, which is clearly not good for classification. On the other hand, in a forced vortex, req is an unstable equilibrium point, because FD and FC pull the particle away from req, assisting in the division of particles at Dpc.
Also contributing to the cut sharpness in a forced vortex is the short residence time of the particles. Because of stable equilibria that form in a free vortex, particles have a longer residence time. This causes particles to accumulate in the classification region, increasing the likelihood that agglomerates will form. In a forced vortex, since particles do not accumulate, agglomeration is less likely.
The principle of counterflow centrifugal classification in a free vortex sink flow has been studied. See Rumpf, H., xe2x80x9cxc3x9cber die Sichtwirkung von ebenen spiraligen Luftstrxc3x6mungen,xe2x80x9d Dissertation, Techn. Hochschule Karlsruhe, Germany (1939). A spiral classifier has been developed based on this principle and sold commercially as the Walther(trademark) classifier. See Rumpf, H. and Wolf, K., Z. VDI, Beihefte Verfahrenstechnik, Vol. 29/38 (1941). The Walther(trademark) classifier has a scroll-shaped housing. Air and particles enter the classifier tangentially and are sent to the classification zone at a controlled angle through eight jets. Air and fine particles exit axially through the center of the classifier. Coarse particles are removed from the air by centrifugal force and are collected in an annular region at the bottom of the classifier. Cut size is controlled by air flow rate.
In another concept for spiral classification tested experimentally to improve cut sharpness, air and particles are supplied together to a cylindrical classifier tangentially through a channel of reducing cross-sectional area to accelerate the air and disperse the particles. See Leschonski, K. and Rumpf, H., Powder Technology, Vol. 2, pp. 175-188 (1968); Weilbacher. M. and Rumpf. H., Aufbereitungstechnik, Vol. 9, No. 7, pp. 323-330 (1968). The height of the channel is the same as that of the classifier. While air and fines exit the classifier through the center, the coarse fraction collects and circulates around the periphery of the classifier. In this way, the coarse fraction is incorporated into the feed stream and re-classified. This process can be continued repeatedly until the coarse fraction is withdrawn through a side vent, which can be opened and closed during operation. By recycling the coarse fraction into the classification region, this classifier can obtain a better separation of coarse and fines at smaller cut sizes than other spiral classifiers. Besides air flow rate, cut size also depends on the volume of material rotating at the periphery of the classifier. As the volume of material increases, the pressure drop decreases, which tends to increase the cut size.
A spiral classifier has been developed that forces the main air stream to pass through a perforated cylindrical wall before combining with the feed. See Metzger, K. L., Richter, W., Schwedes, J., Aufbereitungstechnik, Vol. 20, No. 10, pp. 589-590 (1979). This provides more uniform radial velocity into the classification zone, which is important in achieving a sharp cut. The feed is supplied tangentially to the classifier with a secondary air stream, which adds a tangential component to the air velocity. The diameter of the tube in the center of the classifier through which air and fines exit can be adjusted to control cut size. The coarse fraction exits the classifier tangentially.
In a twin-cone vane classifier described by Luckie, P. T. and Klima, M. S., xe2x80x9cFundamentals of size separation,xe2x80x9d KONA, No. 18, pp. 88-101 (2000) air and particles enter through the bottom of the outer cone of the classifier and travel upward to adjustable guide vanes. The vanes are located above the inner cone in the short cylindrical section of the classifier. Air and particles are accelerated tangentially before passing through the vanes and into the inner cone. Coarse particles are thrown outward to the wall of the inner cone and fall out of the flow. The fine fraction is carried with the air through an exit tube at the top of the classifier. The guide vanes are typically turned at an angle from the radial direction to increase the centrifugal force on the particles. In general, as the vane angle is increased, efficiency increases, and cut size decreases. Cut sizes down to 20 xcexcm have been achieved with the twin-cone, vane classifier. See National Academy of Sciences, Committee on Comminution and Energy Consumption, Report No. 364 (1981).
Gas cyclones are also counterflow centrifugal separators with free vortex sink flow, and have been widely used to remove dust from air. Because of their simple design and inexpensive operation with no moving parts, they are commonly used in industry. Referring to FIG. 1, a conventional gas cyclone, generally designated 10, is illustrated. The standard cyclone design entails a cylindrical section 12 on the top and a conical section 14 on the bottom. Typically, conical section 14 is longer than cylindrical section 12. Air and particles enter gas cyclone 10 at the top tangentially through an inlet 16 with a square or rectangular cross-sectional area, as indicated by arrow A, and swirl within gas cyclone 10 as indicated by arrow B. The coarse fraction is collected at the outer wall of gas cyclone 10 where it falls to the bottom, as indicated by arrow C. Air and fines spiral inward, enter a vortex finder 18 disposed at the top of gas cyclone 10 as indicated by arrow D, and exit gas cyclone 10 as indicated by arrow E. Although mainly used for dust collection and air purification, cyclone separators such as gas cyclone 10 can be used to classify particles. However, because of their turbulent, free vortex flow, gas cyclones do not give as sharp a cut as counterflow centrifugal classifiers with forced vortex flow.
Some free vortex classifiers have rotating walls at the top and bottom of the classification zone. Stationary walls can have adverse effects on classification performance because of decreased flow velocities in this region. A spiral classifier with rotating walls has been developed and sold commercially as Alpine Mikroplex(trademark) classifiers. See Rumpf, H. and Kaiser, F., Chemie-Ing.-Technik, Vol. 24, No. 129, pp. 935 (1952). In the basic Mikroplex(trademark) spiral classifier with rotating walls, air is pulled into a cylindrical, scroll-shaped housing and passes through adjustable guide vanes surrounding the classification zone before combining with the particle feed. Air and particles then enter the classification zone, which has a rotating wall above and below it. The wall rotates at the same speed as the fan that drives the air flow. Air and fine particles exit through the center classifier, while coarse particles are thrown outward and exit through a channel within the periphery of the guide vanes. In the process, the coarse fraction is mixed with the incoming air to remove some of the entrained fines.
In the Mikroplex(trademark) classifier, with or without rotating walls, the characteristic cut size is determined by the classifier diameter. Smaller cut sizes can be achieved as the diameter decreases. However, the maximum throughput allowed to maintain cut sharpness also decreases. The Alpine Mikroplex(trademark) classifier was the first industrial-scale classifier to achieve cut sizes in the range of 10 xcexcm with a reasonably high sharpness of cut. See Nied, R., xe2x80x9cCFS-HD: a new classifier for fine classification with high efficiency,xe2x80x9d International Journal of Mineral Processing, Vol. 44-45, pp. 723-731 (1996). Variations of the Mikroplex(trademark) spiral classifier are available from other manufacturers. In some, the wall rotational speed can be adjusted independently of the fan speed, or a secondary air stream is supplied with the feed stream to more evenly distribute the particles entering the classification zone.
The Bahco(trademark) classifier is a well-known laboratory classifier with rotating walls. See Gustavson, K. E., Teknisk Tidskrift, Vol. 78, No. 10, pp. 667-670 (1948). It is used mainly to determine size distribution of particles in the range of approximately 1 to over 100 xcexcm. The design and operation of the Bahco(trademark) classifier are described in the American Society of Mechanical Engineers, Power Test Code 28: Determining the Properties of Fine Particulate Matter, New York (1985). Air is first pulled radially through a stack of closely spaced rotating disks to impart angular momentum to the air flow. From the disks, air exits axially into the rotating classification chamber. Particles are fed from a hopper above the classification zone using a vibrator and a rolling brush. The particles drop into a vertical tube, where they combine with a high-velocity air stream. The dispersed material falls through the tube into the axis of a narrow channel between two rotating plates. The material is conveyed radially outward through the channel by centrifugal force to a feed slot where it combines with the air stream leaving the rotating disks. Air and particles then enter the classification chamber, which is surrounded by rotating walls. Air and fine particles are pulled radially inward to the outlet at the center. Coarser particles are removed against the air by centrifugal force and are collected in a basin at the outer periphery of the classification zone. The rotational speed of all parts in the Bahco(trademark) classifier is the same as the speed of the fan wheel above the classification zone that drives the air flow. A throttle at the air inlet controls air flow rate. By adjusting the throttle, different cut fractions can be obtained for determining particle size distribution. Because of the principle on which particle size is determined, the Bahco(trademark) classifier is a recommended method for determining the collection efficiency of centrifugal air separators. See EPA Contract No. 68-D-98-026, Work Assignment No. 0-08, Stationary source control techniques for fine particulate matter (October 1998).
The performance of classifiers with a forced vortex is less dependent on particle loading than that of free vortex classifiers, because particle loading cannot diminish the angular momentum of the flow as much in a forced vortex. In essentially all counterflow centrifugal classifiers on the market, a forced vortex is generated by rotating vanes. In the rotor vane, or impeller wheel, classifier, particles dispersed in air pass radially through rotating vanes attached to a rotor. The vanes accelerate the flow tangentially. Coarser particles are collected from the air by centrifugal force. Finer particles are carried with the air through the vanes radially and exit through the center of the classifier. Cut size decreases with increasing rotor vane speed and decreasing air flow rate. Cut sizes for the rotor vane classifier range from about 1 or 2 to 30 xcexcm. Cut sharpness, as well as cut size, tends to decrease as solids loading increases. The different rotor vane classifiers on the market differ mainly in the housing design and the way air and particles are introduced to the classifier.
It has been shown from velocity measurements with a Laser Doppler Anemometer that for radii less than the radius of the inner edge of the rotor vane, the flow becomes free vortex again. See Legenhausen, K., xe2x80x9cUntersuchung der strxc3x6mungsverhxc3xa4ltnisse in einem abweiseradsichter,xe2x80x9d Dissertation, Technical University of Clausthal, Germany (1991). Therefore, air and fines should be withdrawn from the classifier immediately following the inner edge of the rotor vanes to maintain forced vortex flow throughout the classification zone. To achieve forced vortex flow between the vanes, the tangential velocity of the air at the outer edge of the rotor must have the same velocity as the outer edge of the rotor. See Leschonski, K., xe2x80x9cClassification of particles in the submicron range in an impeller wheel air classifier,xe2x80x9d KONA, No. 14, pp. 52-60 (1996). If the velocities are different, vortices will form between the rotors, and the flow field will not be uniform. Typically, it is difficult to match these velocities. See Johansen, S. T. and de Silva, S. R., xe2x80x9cSome considerations regarding optimum flow fields for centrifugal air classification,xe2x80x9d International Journal of Mineral Processing, Vol. 44-45, pp. 703-721 (1996).
An example of a rotor vane classifier is the Acucut(trademark) classifier described by de Silva, S. R., xe2x80x9cCentrifugal air classification for the production of fine powders,xe2x80x9d Journal of Powder and Bulk Solids Technology, Vol. 2, No. 2, pp. 3-12 (1978) and Luckie. P. T. and Klima. M. S., xe2x80x9cFundamentals of size separation,xe2x80x9d KONA, No. 18, pp. 88-101 (2000). In this classifier, air is pulled into the classifier from the bottom in the direction of the axis of rotation at a radius just outside the vanes. Particles enter perpendicular to the air stream and combine with the air before passing through the vanes. The rotor vanes are centrally located in the classifier in the axial direction and occupy nearly the entire height of the classifier. Air and fines exit through a tube at the axis of the classifier, while coarse particles exit tangentially.
Like the free vortex classifiers, many designs for rotor vane classifiers send the coarse fraction back into the incoming air to obtain a sharper cut. In one design tested by Austin, L. G. and Luckie, P. T., xe2x80x9cAn empirical model for air separator dataxe2x80x9d, Zement-Kalk-Gips, Vol. 29, p. 452 (1976), the rotor vanes are located at the top of the classifier. Air is supplied from the bottom. The feed along with a secondary air stream enters the classifier perpendicular to the primary air stream. The combined streams travel upward to the rotor vanes. Air and fines pass though the vanes and exit at the top of the classifier. The coarse fraction falls back into the feed stream and is re-classified.
In the CFS-HD(trademark) classifier, both free and forced vortex flow are used for classification. See Nied, R., xe2x80x9cCFS-HD: a new classifier for fine classification with high efficiency,xe2x80x9d International Journal of Mineral Processing, Vol. 44-45, pp. 723-731 (1996). Stationary guide vanes surround the rotor vanes. Air is fed tangentially from outside the guide vanes, while particles are fed in the space between the guide vanes and the rotor. The purpose of the guide vanes is to create a flow of high shear in the region where particles are introduced to disperse the particles before entering the classification region. The coarse fraction exits tangentially within the periphery of the guide vanes. Air and fines pass through the rotor vanes into an internal, rotor-vane free area inside the rotor, where the flow is free vortex. To achieve more uniform radial velocity in the radial direction in this area, the height of the inside of the rotor was modified to increase as radius decreased. In addition, the air and fines outlet tube was made to rotate in the same direction as the rotor. The rotating outlet extends into the internal area to prevent coarse particles from being carried to the outlet by high radial velocities near the rotor walls. These two design changes resulted in a smaller cut size (d50) for a given geometry and set of operating conditions. Cut sizes as low as 2 xcexcm were achieved, although cut 10 sharpness (K25/75) tended to decrease with decreasing cut size.
The MikroCut MC(trademark) classifier is a recently developed rotor vane classifier that can operate with a centripetal acceleration of over 15,000xc3x97g. See Galk, J., Peukert, W., Krahnen, J., xe2x80x9cIndustrial classification in a new impeller wheel classifier,xe2x80x9d Powder Technology, Vol. 105, pp. 186-189 (1999). Because of this, cut sizes from below 1 to 20 xcexcm for particle densities from 1000 to 4000 kg/m3 can be obtained. Air and particles enter the classifier together tangentially. A secondary air stream of varying flow rate can also be supplied tangentially to the classifier to improve separation of fines from the coarse fraction and to prevent particles from being collected before entering the classification zone. It has been shown that the MikroCut MC(trademark) classifier gives significantly improved cut sharpness over the MikroClassifier CC(trademark) classifier at a solids loading of 0.19 kg solids/kg air for a cut size below 10 xcexcm. The fish-hook effect (a term used to describe the sudden increasing efficiency of particles below a certain size and the resulting appearance of the efficiency curve) was also reduced because of less fine particle agglomeration.
In one study, the effect of the inclination angle of the rotor vanes on classification performance was considered and angles of +30, 0, and xe2x88x9230xc2x0 were tested. See Wang, X., Ge, X., Zhao, X., and Wang, Z., xe2x80x9cStudy on horizontal turbine classification,xe2x80x9d Powder Technology, Vol. 102, pp. 166-170 (1999). It was found that more negative angles gave smaller cut sizes, but more positive angles gave sharper cuts with less fish-hook effect. Therefore, an intermediate angle of 0xc2x0 was recommended. It was believed that the increase in cut size with a more positive inclination angle was due to a transition from laminar to turbulent flow, which lowered the resistance for particles through the classification zone. The increased turbulence also helped to disperse the particles, resulting in higher cut sharpness. A model was developed to predict the fish-hook effect based on extent of agglomeration, which was predicted in terms of Van der Waals force between particles.
A two-phase (gas and solids) model was developed for a rotor vane classifier. See Wang, X., Ge, X., Zhao, X., and Wang, Z., xe2x80x9cA model for performance of the centrifugal countercurrent air classifier,xe2x80x9d Powder Technology, Vol. 98, pp. 171-176 (1998). From this model, an equation was derived for cut size as a function of classifier geometry and operating parameters. Cut sizes obtained by experiment were compared with model predictions. The agreement between experiment and theory was reasonable. The deviation between the experiment and theory was attributed to incomplete dispersion of the particles, a non-uniform force field in the classification zone, the influence of size and shape of the feed material, and experimental error. The model was used to optimize the classifier design and operating parameters for a desired cut size.
A two-phase model was developed for an Acucut(trademark) classifier using the commercial computational fluid dynamics (CFD) code, FLUENT(trademark). See Johansen, S. T., Anderson, N. M., and de Silva, S. R., xe2x80x9cA two-phase model for particle local equilibrium applied to air classification of powders,xe2x80x9d Powder Technology, Vol. 63, pp. 121-132 (1990). The flow was modeled as two-dimensional (assuming the angular velocity of the flow was constant), steady state, and turbulent using a version of the k-E model for two-phase flows. The particle phase was modeled by grouping the particles into a number of size classes. Particle-particle interactions were neglected. Flow fields and particle concentrations with and without particle-gas momentum and turbulence coupling were determined. The efficiency predicted by the model increased when particle-gas coupling was included. It was found that increasing particle concentration reduced the amount of turbulence. In addition, when particle loading was increased above a certain limit, the flow became unstable. To further investigate this phenomenon, it was recommended to model the flow as unsteady-state.
The above-described model was extended to three dimensions. See Johansen, S. T. and de Silva, S. R., xe2x80x9cSome considerations regarding optimum flow fields for centrifugal air classification,xe2x80x9d International Journal of Mineral Processing, Vol. 44-45, pp. 703-721 (1996). The flow between two vanes was modeled using a rotating coordinate system. The flow simulations showed that it was difficult to achieve an ideal flow field for classification between the vanes. Two different inlet gas tangential velocities were considered: one equal to the vane velocity at the outer edge and the other about half this value. In both cases, flow separation occurred but in opposite directions. Addition of particles to the flow increased the amount of flow separation. Predicted efficiency curves were in reasonably good agreement with experimental data.
A problem with the rotor vane classifier is that the vanes tend to impact the particles, especially if the particles do not move at the same velocity as the air. This can cause significant particle attrition if the material being classified is fragile or breaks easily. Particle attrition has been studied in a rotor vane classifier. See Furukawa, T., Ito., M., Fujii, S., and Tanaka, H., xe2x80x9cComminution of particles in centrifugal air classifier,xe2x80x9d Proceedings of 2nd World Congress PARTICLE TECHNOLOGY, Kyoto, Japan, pp. 104-111 (1990). As in the CFS-HD(trademark) classifier, air passed through guide vanes before combining with the particle feed. Using four different feed materials, it was found that attrition increased with increase in rotor speed and air flow rate and with decrease in feed rate and distance between the guide vanes and rotor vanes. Attrition resulted in generation of particles in two size ranges: one in the micron and submicron range and the other in the range between 10 and 100 xcexcm. It was believed that attrition was due to impaction of particles with each other, the wall, and the rotor vanes and turbulence. The increased attrition was also explained by the increased residence time of the particles in the classifier. Because of the decrease of fine particles in the coarse fraction as attrition increases, it was concluded that conditions that lead to attrition also contribute to dispersion.
As discussed hereinabove, some free vortex classifiers, like the Alpine Mikroplex(trademark) classifier have rotating walls at the top and bottom of the classification zone to provide more uniform flow velocities in the axial direction. It has now been demonstrated by Applicants in the present disclosure that the use of rotating disks, instead of rotating walls or rotor vanes, to generate forced vortex flow in a centrifugal air classifier yields significant improvements in classification performance. Since the air flows radially through rotating disks, the disks will not impact the particles as do the rotor vanes. Thus, particle attrition is less.
FIG. 2 illustrates a boundary layer momentum transfer (BLMT) particle separator, generally designated 30, that has been developed by InnovaTech, Inc., and disclosed in U.S. Pat. No. 5,746,789 to Wright et al. as a radial inflow centrifugal filter device for separating fine particulates from fluid flow. The BLMT particle separator 30 uses a stack of closely spaced, rotating, coaxial, annular disks 35 to filter particles from a fluid. These rotating disks 35 are somewhat similar to those in the Bahco(trademark) classifier, except that in the BLMT particle separator 30, particle separation occurs between the rotating disks 35. The disk annulus width is on the order of 25 times that of the disk spacing. Particle-laden fluid enters the device as indicated by arrow F. Since the bottom of the disk stack is closed, fluid is forced radially inward through disks 35 as indicated by arrows G, and exits axially though the center of the disk stack as indicated by arrow H. Momentum is transferred from disks 35 to the fluid in the boundary layer at each disk surface, accelerating the fluid tangentially. This causes coarser particles in the fluid to accelerate by centrifugal force radiallyoutwardly, against the main radially inward flow. Smaller particles are carriedwith the air though disks 35. In accordance with the present invention, it is believed that the BLMT particle separator is well-suited for particle classification in part because of the forced vortex flow imposed by disks 35.
Bickert et al. implemented a principle somewhat similar to that of BLMT particle separator 30 to classify particles in a liquid, as reported in Bickert, G., Stahl, G., Bartsch, R., and Mxc3xcller, F., xe2x80x9cGrinding circuit for fine particles in liquid suspensions with a new counter-flow centrifugal classifier,xe2x80x9d International Journal of Mineral Processing, Vol. 44-45, pp. 531-534 (1996). Their classifier consisted of four, rotating annular disks. Unlike the BLMT particle separator 30, which has uniform disk spacing, the space between the rotating disks increases slightly as the radius decreases. This classifier was able to achieve a wide range of cut sizes for glass spheres by varying the radial fluid velocity, or fluid volumetric flow rate. The experimental cut size was defined as the D99 of the fines, where D99 is the particle diameter for which 99 volume % of the particles are smaller. At solids loading below 5 volume %, experimental cut sizes agreed very well with those predicted by equating the settling velocity in the centrifugal field to the radial fluid velocity. At higher loadings, cut size increased as the solids loading in the classification zone increased. They believe the higher solids loading reduced the settling rate by increasing the viscosity of the fluid. Although particles above the cut size were almost completely separated from those below, the coarse fraction still contained a significant amount of fines. They attributed the poor separation to particles being collected before being introduced to the classification zone. The results in this paper from Bickert et al. did not show the effect of varying centripetal acceleration (disk rotational speed) on cut size.
In view of the foregoing, it can be appreciated that new classification technology is required to meet the existing and future performance demands of the marketplace, particularly in the sharpness of the particle""s cut size. Desired performance criteria include higher efficiencies, smaller size ranges of particles classified, a relatively low pressure drop across the device utilized for classification, and the potential for adaptation (i.e., retrofit) to existing classifiers.
According to the invention, a novel centrifugal classifier apparatus is provided that takes advantage of the boundary layer momentum transfer (BLMT) concept to greatly improve the efficiency of fine particle classification with lower particle attrition of friable particles. The classifier apparatus of the invention is durable, self-cleaning, and highly efficient across a wide range of particle sizes, thereby resulting in improved performance and reduced maintenance. In addition, the system parameters associated with the classifier apparatus are easily variable so that a wide range of particle sizes can be classified from the inlet air stream.
According to the invention, the classifier apparatus is provided with a BLMT particle separator assembly comprising rotating disks, and preferably with a novel inlet device as described hereinbelow in the form of several different embodiments. In this classifier apparatus, a particle-carrying fluid such as air is forced to flow radially inward between closely spaced, rotating, coaxial, annular disks. The disks accelerate the air tangentially, causing coarser particles to be removed by centrifugal force. Finer particles are carried with the air through the disks by the drag force exerted by a fluid moving device such as a blower situated either upstream or downstream of the BLMT particle separator device. Hence, the centrifugal force imparted to both the fluid and the particles causes the fluid and particles to be separated in opposite directions. Moreover, the centrifugal force is oriented in a direction opposite to that of the drag force. The classifier apparatus of the present invention differs from other forced vortex centrifugal classifiers in that it does not supply rotational momentum by collision of the particles with rotating vanes. Thus, classifier apparatus of the present invention is more effective in separating fragile particles without breaking them.
More specifically, the BLMT particle separator provided by the invention comprises a hollow-core stack of disks that can be rotated at several hundred to several thousand rpm. In combination with a fluid moving device such as a blower situated either upstream or downstream of the BLMT particle separator, the fluid pressure at the core of the BLMT particle separator is reduced and, consequently, outside fluid is drawn through the rotating disks. The rotation of the disks establishes a boundary layer on each side of every disk in the stack, thereby imparting rotation to the fluid and particles in the incoming fluid stream. A pressure drop across the disk stack from its outer edge to its inner edge is caused by the frictional losses of the outside fluid traversing through or near the boundary layer. The boundary layers and the pressure drop across the disk stack can be affected by a number of parameters, including disk size, the spacing between each disk, disk rotational speed, downstream pressure, ambient fluid conditions, and the like. Particle-laden fluid enters the classifier apparatus, enters the disk stack from the perimeter thereof, exits the disk stack through an open end thereof, and exits the classifier apparatus through an appropriate outlet.
When the outwardly directed centrifugal force balances the inwardly directed drag force on the particles, the particle cut size for the classifier apparatus is achieved. Angular momentum transfer from the rotating disks, effected by means of the intra-disk boundary layers established in the BLMT particle separator device, causes any particles above the cut size selected for the classifier apparatus that are entrained in the incoming fluid to be immediately expelled away from the perimeter of the BLMT particle separator device. The expelled particles are preferably retained in a collection component disposed below the BLMT particle separator device. As indicated hereinabove, particles smaller than the cut size follow the fluid streams through the rotating disks, into the central plenum defined by the disks, and out from the classifier apparatus, thereby effecting separation and classification of the particles of the incoming fluid stream.
The rotational requirement for the classifier apparatus of the invention can be provided by several different means, such as a small external electric or air-driven motor or other power means. Rotational power requirements are minimal due to relatively low boundary layer drag losses; rotating flat disks can easily sustain constant or substantially constant velocity with little power drain once accelerated to an appropriate operational speed.
The effects of disk rotational speed and inlet geometry, which effectively controls the tangential velocity of the air surrounding the disks, on classification performance were evaluated on embodiments of the classifier apparatus described in detail hereinbelow. Embodiments of the classifier apparatus have given sharp cuts for a wide range of cut sizes with both ideal glass spheres and non-ideal, industrial particles. It was initially expected that cut size would decrease monotonically with increasing disk speed because of higher centrifugal force on the particles. However, over a certain range of disk speeds, cut size increased with increasing disk speed. Further analysis demonstrated that disk rotation causes flow near the disk surface to reverse. To compensate for the reversed flow, the velocity radially inward at the center between the disks increases, increasing the drag force, which could explain the observed increase in cut size with increasing disk speed. As disk speed increases further, the flow becomes turbulent. Turbulence makes the velocity profile more uniform, reducing the reversed flow and thus the center radial velocity inward. More uniform velocity profiles are also in agreement with sharper cuts observed at higher disk speed.
According to one embodiment of the present invention, a radial inflow centrifugal apparatus for classifying a mixture of fine and coarse particles in a fluid stream by size or density comprises a housing, a boundary layer momentum transfer device, an inlet flow control mechanism, and a drive mechanism. The housing comprises an inlet, an interior chamber, a coarse particle outlet, and a fine particle outlet. The boundary layer momentum transfer device comprises a plurality of disks stacked in spaced, parallel relation in the interior chamber. The disks are rotatable about a disk axis. The disks have respective central openings cooperatively defining a plenum having a closed axial end and an opposing open axial end. The plenum communicates with spaces defined between each adjacent disk to cooperatively define a fine particle flow path from the interior chamber, through the spaces, through the plenum and the open axial end thereof, and to the fine particle outlet. The inlet flow control mechanism communicates with the interior chamber and provides an adjustable inlet flow path into the interior chamber. The drive mechanism is coupled to the boundary layer momentum transfer device to cause rotation of the disks.
According to a method of the present invention, a mixture of fine and coarse particles in a fluid stream are classified, wherein fine particles have a size or density below a predetermined cut size and coarse particles have a size or density above the cut size. A particle separation assembly is provided. The assembly comprises a housing and a boundary layer momentum transfer device. The housing comprises an inlet, an interior chamber, a coarse particle outlet, and a fine particle outlet. The boundary layer momentum transfer device comprises a plurality of disks stacked in spaced, parallel relation in the interior chamber, and which are rotatable about a disk axis. The disks have respective central openings cooperatively defining a plenum having a closed axial end and an opposing open axial end. The plenum communicates with spaces defined between each adjacent disk. A decreasing pressure gradient from the disk spaces to the plenum is created to establish a fine particle flow path through the inlet of the housing, the interior chamber, the disk spaces, the plenum, the open axial end of the plenum, and the fine particle outlet. A particle-laden fluid stream is flowed through the inlet of the housing into the interior chamber. A tangential velocity component of the fluid stream adjusting to promote uniform dispersion of particles in the fluid stream as the fluid stream flows around and toward the disks. The disks are rotated to eject coarse particles away from the disks, whereby fine particles are permitted to continue along the fine particle flow path to the fine particle outlet.
It is therefore an object of the present invention to provide a radial-flow, vaneless, centrifugal classifier apparatus characterized by novel structural features enabling advantageous particle classification performance.
It is another object of the present invention to provide a classifier apparatus characterized by improved discrimination of very fine particles over state-of-the-art classifier designs.
It is still another object of the present invention to provide a classifier apparatus and method that generate boundary layers on parallel, rotating disks to transfer momentum to particles suspended in an incoming flow of air rather than vanes.
It is yet another object of the present invention to provide a classifier apparatus that comprises a filtering device presenting no physical impediments or obstructions to fluid flow therethrough, thereby resulting in a desirable low pressure drop across the filtering device, which in turn reduces fluid moving energy demand and cost.
It is a further object of the present invention to provide a classifier apparatus that is inherently non-fouling, thereby significantly reducing requirements of maintenance.
It is a still further object of the present invention to provide a classifier apparatus that is mechanically simpler than vane-type classifier designs, and thus having a minimum of components, and which components are relatively low in cost, thereby further reducing the cost of maintenance and increasing economic life.
It is a yet further object of the present invention to provide a classifier apparatus and method capable of selectively removing particles from an incoming fluid stream based on the size or density of the particles by judicious choice of operating parameters that can be varied to change classification performance.
It is an additional object of the present invention to enhance classifier performance by providing a containment housing that includes a cyclone, cylinder, or scroll design.
Some of the objects of the invention having been stated hereinabove, other objects will become evident as the description proceeds when taken in connection with the accompanying drawings as best described hereinbelow.